Hydraulic machines



May 14, 1963 D. FIRTH ETAL 3,089,427

HYDRAULIC MACHINES Filed April 14. 1960 7 Sheets-Sheet 1 Donald Firth 8Roger Harvey Yorke Hancock BY R'RL- C/Q-IFLOCKS ATTORNEY May 14, 1963 D.FIRTH ET AL HYDRAULIC MACHINES 7 Sheets-Sheet 2 Filed April 14. 1960INVENTORS Donald Firth 8 Roger Harvey Yorke Hancock BY ,(RRL Ulq'Io CKSATTORNEY May 14, 1963 D. FIRTH ET AL HYDRAULIC MACHINES 7 Sheets-Sheet 3Filed April 14. 1960 INVENTORS Donald Firth 8 Roger Harvey Yorke HancockBY I QRL (Alma/ 5 ATTORNEY y 1963 D. FIRTH ET AL 3,089,427

HYDRAULIC MACHINES Filed April 14. 1960 7 Shets-Sheet 4 INVENTORS DonaldFirth a Roger Harvey Yorke Hancock av 6721. (4.).1LocK5 ATTORNEY May 14,1963 Filed April 14. 1960 7 Sheets-Sheet 5 LIE-z 22a 30 27 T T L \1 9 229 l -/0" l 6 i Z2 6/ /a v I7 INVENTORS Donald Firth 8 Roger HarveyYorke Hancock BY MW ROCKS ATTORNEY y 1963 D. FIRTH ET AL 3,089,427

HYDRAULIC MACHINES Filed April 14. 1960 '7 Sheets-Sheet 6 INVENTORSDonald Firth 8\ Roger Harvey Yorke Hancock ATTORNEY United States Patent3,689,427 HYDRAULIC MACHWES Donald Firth and Roger H. Y. Hancock, EastKilbride, Glasgow, Scotiand, assignors to Council for Scientific andindustrial Research, London, England, a corporation of the UnitedKingdom Filed Apr. 14, 196i), Ser. No. 22,334 Claims priority,application Great Britain Apr. 16, 1959 13 Claims. (Cl. 103-462) Thisinvention relates to hydraulic machines, and particularly to swash platetype pumps and motors. Hitherto, in pumps of the swash plate type inwhich the cylinder block is rotated, considerable difliculty hasfrequently been encountered in developing sutliciently high pressureswithout risk of distortions or deflections of the cylinder blocksufficient to cause seizure between it and the port or valve block orplate if the working clearances are kept to the very small valuesnecessary to avoid undue leakage. The usual steps taken to combat thesedifliculties have hitherto been to increase the overall diameter of thecylinder block so as to impart a higher resistance to distortion, and toemploy a flexible drive shaft or spherical seatings between the cylinderblock and the port or valve plate for maintaining the required smallworking tolerances. The former leads to high inertia, andcorrespondingly sluggish response to signals, and the latter tomechanical complication.

Furthermore, it has usually been found necessaryin order to maintain thenecessary fine working tolerances and to reduce friction losses-tosupport the cylinder block in comparatively large ball or rollerbearings.

It is an object of the present invention to avoid or reduce to a minimumthe difficulties hitherto experienced in the design and operation ofswash plate pumps with rotary cylinder blocks without the disadvantagesof high inertia and friction or substantial risk of seizure at highoutput pressures.

A further object is to provide a construction which embodies a highdegree of automatic hydrostatic balance between the mechanical and oillubricant pressures at the working surfaces of the port plate so thatfriction losses are kept very low.

By arranging the parts according to the present invention so as to makeuse of the principle of hydrostatic balance by means of oil filmsbetween the various working surfaces, it has been found possible toproduce a design of pump which is not only of relatively small overalldimensions with low frictional drag, but also one which embodies simpleand inexpensive plain journal bearings for supporting the shaft of thecylinder block.

A principal feature of the present invention is the interposition,between the working face of the cylinder block and a ported mountingblock or end plate of the pump carrying the inlet and deliveryconnections, of an annular port plate concentric with the cylinder blockshaft and whose opposite faces are at all times substantially supportedon oil films the pressure of which is a function of the mechanical loadimposed on the plate by the compression of the oil or other hydraulicworking fluid in the cylinder block.

The port plate has a limited degree of radial float which is controlledby radially opposed hydrostatic pressures applied between the port plateand a relatively stable radially locating surface on the machine so asto vary differentially with variation of the clearance between the portplate and the said reference surface.

Conveniently, the opposed hydrostatic forces governing the radial fioatof the port plate act between the inner periphery of the annular plateand the radially stable surface of the shaft, the said periphery of theplate being formed with oil retaining cavities or pockets symmetrical-1y arranged and connected to a source of oil under pressure so as toproduce radially opposed stabilising oil pressures between the portplate and the shaft the difference between which at any instant is afunction of any radial force tending to displace the floating port plateradially and cause metal to metal contact between it and the shaft.

If preferred, the radially opposed stabilising oil pressures on the portplate may be applied at the periphery of the port plate by locating theplate in a recess in the adjacent surface of the frame or end cover ofthe machine, and forming the pressure oil cavities or pockets either inthe circumferential edge of the port plate or in the surrounding wall ofthe recess.

All the oil cavities or pockets may conveniently be interconnected by acommon channel or .groove which in turn is fed with oil under pressurefrom the high pressure port of the machine.

The necessary radial floating action of the port plate depends on theachievement of a significant reduction in oil pressure in a cavity orpocket whenever the clearance between the port-plate and its relativelystable radially locating surface increases in the vicinity of the cavityor pocket Thus, each pocket must be connected to the source of oil underpressure through a narrow duct or constriction across which a rapidpressure drop is built up as the rate of flow tends to rise.

A hydraulic swash plate pump according to the present invention iscapable of high power outputs-cg. 200 H.P.-at speeds which are variableover a wide range and in which rapid response can be obtained to inputcontrol signals. The reductions possible in bearing surface areas reducefrictional loss, and. hence friction heat, in the working parts.

A preferred embodiment of the invention will now be particularlydescribed, by way of example only, with reference to the accompanyingdrawings in which:

FIGURE 1 is an axial cross section of the complete pump assembly;

FIGURE 2 is an elevation of the discharge end of the FIGURE 3 is afragmentary section on the line HIIII of FIGURE 2;

FIGURE 4 is a fgragmentary side elevation as seen on the arrow IV inFIGURE 2;

FIGURE 5 is a section on the line V--V of FIGURE 4;

FIGURE 6 is a diagrammatic sectional view of the port plate arrangement;

FIGURE 7 is a cross section on the line VII-VII of FIGURE 6;

FIGURE 3 is a sectional view on the line VIIIVIII of FIGURE 7 andFIGURES 9 and 10 are sections similar to FIGURE 7 showing modifiedarrangements.

The pump illustrated consists of a main frame or body having front andported back end plates 1, 2 clamped by four pillars 3 (FIGS. 2, 3 and5). Each end plate 1, 2 carries a journal bearing 4-, 5 respectively fora short rigid drive shaft '6. Adjacent the bearing 5 in the back endplate 2, the shaft 6 is formed with a locking taper section 7 on whichis locked a cylinder'block 8. This block is drawn up on the taper by aback-nut 9 on the shaft. The cylinder block 8 contains a number ofcylinde'rs 10 whose axes are mutually inclined inwards towards the backend plate 2. A piston 11 in each cylinder is reciprocable under thecontrol of a normally fixed swash plate '12 carried on trunnions 13(FIG. 5 The swash plate 12 has a central conical aperture 14 throughwhich the shaft 6 passes, the dimensions of this aperture beingsufficient to allow for adjustment of the angle of the swash plate tothe shaft 6.

The working face of the swash plate is recessed to accommodate anannular bearing pad 15 and an annular slipper plate 16. The latter isfree to rotate under the frictional drag of slippers 17 each of which isengaged with a respective piston 11. For clarity of illustration in FIG.1, only one cylinder 10, piston 11 and slipper 17 is shown.

The back face 18 of the cylinder block 8 (see FIG. 6) is pierced byinlet/ outlet ducts 19, one to each cylinder 10, the openings to whichregister successively with arenate inlet and outlet ports 29, 21 in afloating port plate 22. This port plate is not anchored to the back endplate 2 except for angular orientation about the shaft 6 with respect tothe swash plate 12. This angular orientation is controlled by means of athreaded rod 23 which is slidable in a bore 24 in the back end plate 2and which engages, at its inner end, a locating peg 25 fastened to theport plate 22 and projecting into the bore 24. The rod 23 is axiallyadjustable by means of a captive capstan nut 26. The ports 20, 21communicate with an external circuit by way of flared ducts 27, 28respectively (see FIGS. 1 and 2) and inlet and outlet sockets 27a, 28ain the back end plate 2. The ports 27, 28 open through a bearing surface29 on the back end plate to register with the arcuate ports 20, 21,respectively in the port plate 22.

The port plate 22 forms a kind of washer between the mutually opposedfaces 18 of the cylinder block 8 and 29 of the back end plate 2, and hasa limited freedom to float radially between the two. This radialfloating action is hydrostatically controlled by means of internal ductsand cavities.

As shown best in FIGS. 7 and 8, the port plate 22 has a central insertor hub 22a which fits over the shaft 6 with slight radial clearance orfloat. Around the internal periphery of the hub 22a are four equallyspaced oil pockets 30, 31, 32, 33. Each pocket communicates with anouter peripheral groove 34 around the hub 22a by means of small-boreradial ducts 35, 36, 37, 38 respectively, and this peripheral groove isin turn directly connected to the main arcuate outlet port 21 'by meansof a radial bleed passage 39. Thus, each oil pocket 30-33 is fed withoil at pump delivery pressure, and since the pockets are cquiangularlyspaced around the internal pe riphery of the hub 22a, the radial forcesexerted by the oil pressures therein tend to balance out and maintainthe port plate 22 centralised with respect to the shaft 6. In thisfloating position, there is a narrow leakage path for oil between theinner periphery of the hub 22a and the radially stable locating surfaceof the shaft 6, and all leakage paths are of the same nominal size andhence are of equal flow resistance. There is, therefore, a continuousflow of oil through the bleed passage 39 into the groove 34, and fromthis groove along each radial duct -38. The pressure drops along theseparallel paths are equal.

Let it now be assumed that the plate 22 is radially displaced withrespect to the shaft 6 in the vertically downward direction in FIG. 7.The leakage paths from the pockets 30 and 31 are then restricted whilstthose from the pockets 32 and 33 are increased. Less oil will thenescape from the pockets 30 and 31, and more will escape from the pockets32 and 33. Hence, the pressure in the pockets 30 and 31 will rise andthat in the pockets 32 and 33 will fall, tending to restore the plate22. to its centralised position in which friction with the shaft 6 is ata minimum.

Instead of being radially located on the shaft 6, the port plate 22 maybe located at its outer periphery by an annular wall on the back endplate 2 of the machine casing, which thus constitutes the relativelystable or reference surface. An increased radial clearance is thenprovided between the port plate 22 and the shaft 6 so as to avoid anyrisk of binding of the plate on the shaft. Alternative constructions ofthis modified arrangement are illustrated in FIGS. 9 and 10.

In FIG. 9, the port plate 22 is shown as having a peripheral groove 34amachined around its circumference which is connected to the pressureport 21 by an internal duct 39a drilled radially in the thickness of theplate. This groove is closed by a ring 122 shrunk onto the plate 22 toform a rim. The ring has four pockets 30a, 31a, 32a and 33a at spacingsaround its circumference, these pockets being connected to the groove34a by ducts 35a, 36a, 37a and 38a and corresponding in function to thepockets 3033 described above with reference to FIGS. 7 and 8. Theperiphery of the rim 122 is located with small clearance within anannular wall 2a bounding the surface 29 on which the port plate 22rests. The action of the oil in the pockets 30a-33a and the narrowclearance from the wall 2a results in the same kind of self-centerin gaction as occurs at the shaft 6 in the arrangement of FIGS. 7 and 8.

FIG. 10 illustrates the same principle as FIG. 9 except that the pockets30b, 31b, 32b and 3312 are now formed in the wall 2a which constitutesthe inner periphery of a ring 222 which is a force fit in a recess inthe back end plate 2. A circumferential groove 34b is machined aroundthe outer circumference of the ring 222 and com municates with eachpocket 30b, 31b, 32b and 33b by ducts 35b, 36b, 37b and 38b. It alsoregisters with a supply duct 39b leading from the high pressure port 28.The action of this arrangement is identical with that of FIG. 9, as willbe understood.

When the pump is working, half the total number of pistons 11 are beingforced inwards against high oil pressure by the swash plate 12 whilstthe other half are being forced outwards by low pressure oil. Since thethrust on each piston is taken through a slipper 17 on an inclinedsurface '16, there will be a lateral component of reaction to eachthrust which will be exerted on the respective cylinder wall and sincethese reactions are unequal over the two diametrically opposed arcsdefined by the outlet and inlet ports 21, 20 respectively, the cylinderblock 8 experiences a tilting couple about the horizontal plane inFIG. 1. This couple tends to close the gaps at G G (FIG. 6) between theport plate 22 and the adjacent faces 18 of the cylinder block 8 and 29of the back end plate 2 over an are on the inlet side and at the sametime to open thediamet-rically opposite gaps at G G This couple isresisted by the short, stout shaft 6, and its magnitude is kept to aminimum by the reduction in overall diameter of the port face 18 of thecylinder block 8 which results from the inclination of the axes of thecylinder 10, so that a frequent source of seizure or loss of output inpumps of this general kind, is accordingly minimized.

The floating port plate 22 itself also assists in mitigating possibletrouble due to distortion of the cylinder block 8 under load. Althoughgaps are indicated in FIG. 6 at 6 -6 it will be understood that theirdimensions are of the order of a few ten-thousandths of an inch.However, they exist physically and serve partly to cushion oraccommodate cylinder block deflections and partly to combat distortionof the port plate 22 under fluctuating load conditions. The gaps areinitially formed with the aid of capillary action.

Referring again to FIG. 6, the gaps G G are intersected by that part ofthe inlet circuit comprising the passage 27, port 20, and the relevantcylinder inlet/outlet ducts 19. Similarly, the gaps G G are intersectedby that part of the outlet circuit comprising the relevant cylinderinlet/outlet ducts 19, port 21, and outlet passage 28. In each of thesecases, the oil in the circuit is at a pressure above ambient pressure inthe pump casing, the cylinders 10 being charged by oil at a positiveinlet pressure from an auxiliary pump. Hence, leakage will tend to takeplace at both surfaces of the port plate 22, as indicated by the arrowsL in the gaps G G and L in the gaps G G Since the ports 20, 21 are verylarge compared with the dimensions of the gaps G G the leakage pressuredrop across the plate 22 is, for all practical purposes, zero, so thatat all points around the plate the pressures on opposite faces areequal, although the pressures on either face of the plate at differentpoints around the shaft 6 5. vary from maximum pump delivery pressure tominimum inlet pressure. This range of variation may be from 2000 lbs/in.to 50* lbs/m Provided that there is radial clearance or float betweenthe port plate 22 and the shaft 6, as described with reference to FIGS.7 and 8, the port plate 22 floats axially irrespective of the actualvalue of pump delivery pressure, and the system is self-stabilising.Most of the couple tending to cause tilt of the cylinder block 8 due touneven pressure loading is resisted by the short, rigid drive shaft 6,and any residual deflection can be accommodated by compression of theleakage oil layers in the gaps G and G The aperture surface areas onopposite sides of the floating port plate 22 are proportioned so as toleave a relatively small residual unbalanced axial thrust on the shaft 6which serves to overcome any tendency for the port plate 22 and cylinderblock 8 to float axially in an uncontrolled manner leading toinstability, and breakdown of the oil films. Due to the cyclic nature ofthe operation of successive pistons in each revolution of the cylinderblock, a certain degree of pumping action is generated which tends tokeep the films continuous, and this can, by careful design, be utilisedto keep the mean unbalanced load between the plate 22 and the end plate2 sufficiently low to prevent the port plate from binding on the endplate.

The combination of inclination of the cylinder axes and a fully floatingport plate 22 materially reduces the frictional drag on the shaft 6. Theradial width of the annular face 18 of the cylinder block 8 determinesthe total force due to the pressure of leakage oil acting on thecylinder block. This force is balanced by the force acting on the areaof the end wall of each pumping cylinder 1% around the outlet duct 19.

The smaller the pitch circle diameter of the ports 20, 21, the less isthe effective radius at which leakage oil in the gaps G and 6;; of FIG.6 is sheared, and the lower is the power loss absorbed thereby. On theother hand, if the amount of leakage at these gaps is tobe kept withinpracticable limits, the difference in radius of this pitch circle andthe outer circumference of the face '18 and port plate 22 must be aslarge as possible in order to give along leakage path of highresistance.

All these factors influence the design of the cylinder block 8 and portplate 22, and determine the angle of inclination of the cylinders to theaxis of the shaft 6.

The provision for angular adjustment of the port plate 22 enablessmoother operation of the pump to be obtained than is possible withconventional swash plate pumps in which no such adjustment is possible.The reason for this is that, since the pistons 11 cannot suck oil intotheir cylinders lil, charging of the cylinders through the inlet port 20must be effected under positive pressure. This must necessarily be lowin comparison with the delivery pressure, so that at T.D.C. of thepiston 11 (i.e. the completion of the charge stroke), the cylinder 10contains oil at low pressure. If now the common inlet/outlet duct 19 isopened to the delivery port 21, a pressure surge will be experienced inthe cylinder due to the much higher pressure in the load circuit, and aseries of shock waves would be set up in the system each time a pistoncommenced its delivery stroke. Hence, a small angular la-g must beprovided between T.D.C. and the opening of the duct 19 to the deliveryport 21 to allow the oil to be pre-compressed in the cylinder 16 to atleast approximately line pressure. The correct setting of the angularadjustment rod 23 which controls the port plate setting may be obtainedby trial and erroras by means of the captive capstan nut 26or may bedetermined by a servo mechanism responsive to load on the pump. In thislatter event, the fully floating feature of the port plate 22considerably reduces the effort required by the servo motor, so that amore rapid response to changes in load can be achieved, with consequentimprovement in the smoothness of running.

The swash plate 12, as already noted, is mounted on trunnions 13, andthese are carried in bushes 42, 43 (FIG. 5) mounted in side walls 44, 45of the pump casing. The bushes 42, 43 are locked in position by means ofcap screws 46.

Since the swash plate '12 takes the whole of the reaction of each piston11, it will experience both the cyclic variations in load on each pistonand also any shocks due to mis-timing of the outlet or delivery port 21.Vibrations will, therefore, be set up in the swash plate 12 which arepreferably absorbed by some convenient damping means (not shown) andoperating either internally (i.e. within the structure of the swashplate assembly) or externally on any of the conventional principles,such as a dashpot, boundary lubrication, viscous or dry friction,inertia, or a combination thereof.

The angle of tilt of the swash plate 12 is adjusted by means of an arm47 FIG. 4) locked on the projecting end of one trunnion 13, theextremity of which is clamped between a pair of adjustment screws 48threaded through brackets 49 bolted on the side wall 42. Each screw 48has a knurled head 50 and carries a knurled locking nut 51. A pointer 52projecting from the boss of the arm 47 registers with a scale 53graduated in degrees of tilt of the swash plate 12.

The working face of the swash plate is constituted by the freelyrotatable circular slipper plate 16 which beds against the bearing pad15 in a circular recess in the swash plate. The bearing pad 15 is lockedagainst rotation by means of a dowel. Thus, the friction between thepiston slippers 17 and the slipper plate 16 causes the latter to tend tofollow the slippers 17 around the shaft 6, whilst the eccentricity ofthe plate due to the angle of tilt of the swash plate 12 causes theslippers 17 to trace a path over the working surface of the slipperplate which is not of constant configuration. Thus wear of the plate isdistributed over an area greater than the annulus traced by a singleslipper 17 if the plate 16 were stationary.

In a practical hydraulic transmission system embodying the invention,the speed ratio between the drive for the pump and the motor operated bythe pump is in finitely variable over a relatively wide range byadjustment of the angle of tilt of the swash plate 12. The arm 47 (FIGS.4 and 5) in such an arrangement is linked to a speed control instead ofbeing clamped in any given position of angular adjustment by the screws48. This speed control may be either manual or automatic in response tosome external variable associated with the motor output, according tothe requirements of the case.

It is to be understood that, with appropriate design modifications, thepump described above may be used as a motor.

We claim:

1. A swash plate type hydraulic machine comprising a stationary swashplate, a rotatable cylinder block, a plurality of pistons reciprocablein cylinders in said cylinder block under control of the swash plate,ports in a flat face of the cylinder block communicating with thecylinders therein, a stationary port block having a flat face, inlet andoutlet ports in said fiat face of the port block registrable with theports in the cylinder block, a port plate interposed beween the flatfaces of the cylinder block and the port block and having aperturespermitting the passage of fluid between the ports in the cylinder blockand the ports in the port block, said port plate having opposite flatfaces of substantially equal area and clearances being provided betweensaid opposite faces and the flat faces of the cylinder block and theport block within the limits of which clearances the port plate is freeto move axially, whereby pressure fluid from said ports maintains afluid film on each flat face of the port plate giving substantiallyequal and opposite axial loadings, a radial locating member having anannular surface spaced with radial clearance from an annular surface onsaid port plate within the limits of which clearance the port plate isfree to move radially, a plurality of pockets disposed symmetricallyaround one of said annular surfaces and open toward the other annularsurface and a constricted fluid-supply connection to each of saidpockets.

2. A machine according to claim 1 wherein the fluidsupply connectioncomprises a passageway having constricted branches each leading to oneof said pockets.

3. A swash plate type hydraulic machine comprising a stationary swashplate, a rotatable cylinder block, a plurality of pistons reciprocablein cylinders in said cylinder block under control of the swash plate,ports in a flat face of the cylinder block communicating with thecylinders therein, a stationary port block having a flat face, inlet andoutlet ports in said flat face of the port block registrable with theports in the cylinder block, a shaft carrying said cylinder block, anannular port plate disposed coaxially around said shaft and interposedbetween the flat faces of the cylinder block and the port block andhaving apertures therethrough registering with the ports in the portblock, said port plate having opposite flat faces of substantially equalarea and clearances being provided between said opposite faces and theflat faces of the cylinder block and the port block within the limits ofwhich clearances the port plate is free to move axially, wherebypressure fluid from said ports maintains a fluid film on each flat faceof the port plate giving substantially equal and opposite axial loadingsthereto, a plurality of pockets spaced symmetrically around the innerperiphery of said port plate and open toward the shaft, a radial leakageclearance being provided between the shaft and the port plate within thelimits of which clearance the port plate is free to move radially, and apressure-fluid connection from at least one of the aforesaid ports inthe port block to said pockets through constrictions each associatedwith a separate pocket.

4. A swash plate hydraulic machine comprising a stationary swash plate,a rotatable cylinder block, a plurality of pistons reciprocable incylinders in said cylinder block under control of the swash plate, portsin a flat face of the cylinder block communicating with the cylinderstherein, a stationary port block having a flat face, inlet and outletports in said flat face of the port block registrable with the ports inthe cylinder block, a port plate interposed between the flat faces ofthe cylinder block and the port block and having apertures permittingthe passage of fluid between the ports in the cylinder block and theports in the port block, said port plate having opposite fiat faces ofsubstantially equal area and clearances being provided between saidopposite faces and the flat faces of the cylinder block and the portblock within the limits of which clearances the port plate is free tomove axially, whereby pressure fluid from said ports maintains a fluidfilm on each flat face of the port plate giving substantially equal andopposite axial loadings, a radial locating member having a circularrecess into which the port plate fits with a radial clearance betweenthe circumferential surfaces of the port plate and the recess, aplurality of pockets spaced symmetrically around one of saidcircumferential surfaces and open toward the other circumferentialsurface, and a constricted pressure-fluid connection to each of saidpockets.

5. A machine as claimed in claim 4 wherein said pockets are disposed inthe port plate.

6. A machine as claimed in claim 4 wherein said pockets are disposed inthe radial locating member.

7. A swash plate type hydraulic machine comprising a. stationary swashplate, a rotatable cylinder block, a plurality of pistons reciprocablein cylinders in said cylinder block under control of the swash plate,ports in a flat face of the cylinder block communicating with thecylinders therein, a stationary port block having a flat face, inlet andoutlet ports in said flat face of the port block registrable with theports in the cylinder block, a port plate interposed between the flatfaces of the cylinder block and the port block and having aperturespermitting the passage of fluid between the ports in the cylinder blockand the ports in the port block, said port plate having opposite flatfaces of substantially equal area and clearances being provided betweensaid opposite faces and the fiat faces of the cylinder block and theport block within the limits of which clearances the port plate is freeto move axially, whereby pressure fluid from said ports maintains afluid film on each flat face of the port plate giving substantiallyequal and opposite axial loadings, a radial locating member having anannular surface spaced with radial clearance from an annular surface ofsaid port plate within the limits of which clearance the port plate isfree to move radially, a plurality of pockets at equally spaced pointsaround the annular surface of the port plate and opening toward theannular surface of the radial locating member, and a constrictedpressure-fluid connection to each of said pockets from an aforesaidport.

8. A swash plate type hydraulic machine comprising a housing having anend plate with a flat surface, inlet and outlet ducts in said end platefor conveying hydraulic fiuid, a rotatable shaft mounted in said housingon an axis normal to said flat surface, a swash plate mounted in saidhousing on said axis, a cylinder block fixed on said shaft and locatedbetween said end plate and said swash plate, cylinders in said blockdisposed around said axis, a port communicating with each of saidcylinders and opening through a fiat end face of said cylinder blockopposite to said flat surface of the housing endplate for conveying saidfluid to and from the cylinder, a reciprocable piston in each cylinderdisplaceable by said swash plate on relative rotation of said swashplate and said cylinder block, an annular port plate whose oppositesides have smooth flat surface of equal area, the port plate beingmounted to float axially between said flat surface on the housing endplate and said fiat end face of the cylinder block and to float radiallywith respect to said axis, pressure balancing means for limiting radialmovement of the port plate, spaced arcuate transfer ports in said portplate respectively registering with said inlet and outlet ducts in thehousing end plate and registrable in sequence with said ports in thecylinder block on rotation of the cylinder block, whereby fluid can betransferred by reciprocation of said pistons from the inlet duct, viasaid transfer ports and said ports and cylinders in the port block, tothe outlet duct, said pressure balancing means comprising a plurality ofpockets opening through an annular surface of the port plate andsymmetrically disposed about said axis, a pressure-fluid connection toeach pocket, a restriction in each pressure-fluid connection whereby thepressure in each pocket depends on the rate of escape of fluidtherefrom, a radial locating member radially fixed relative to said axisand having a circular reference surface adjacent to and spaced from saidannular surface of the port plate for controlling the rate of escape offluid from said pockets in dependence on the spacing of said annularsurface from the reference surface.

9. A machine according to claim 8 wherein said radial locating membercomprises said shaft.

10. A machine according to claim 8 wherein said radial locating membercomprises said casing and said circular reference surface is provided bythe wall of a recess in the casing.

11. A machine according to claim 8 wherein said pressure-fluidconnections lead to one of said arcuate transfer passages.

12. A machine according to claim 8 wherein the port plate is angularlyadjustable relative to the swash plate.

13. A swash plate type hydraulic machine comprising a housing having anend plate with a flat surface, inlet and outlet ducts in said end platefor conveying hydraulic fluid, a rotatable shaft mounted in said housingon an axis normal to said flat surface, a swash plate mounted in saidhousing on said axis, a cylinder block fixed on said shaft and locatedbetween said end plate and said swash plate, cylinders in said blockdisposed around said axis, a port communicating with each of saidcylinders and opening through a flat end face of said cylinder blockopposite to said flat surface of the housing end plate for conveyingsaid fluid to and from the cylinder, a reciprocable piston in eachcylinder displaceable by said swash plate on relative rotation of saidswash plate and said cylinder block, an annular port plate Whoseopposite sides have smooth flat surfaces of equal area, the port plateeing mounted to float axially between said flat surface on the housingend plate and said flat end face of the cylinder block and to floatradially with respect to said axis, pressure balancing means forlimiting radial movement of the port plate, and spaced arcuate transferports in said port plate respectively registering with said inlet andoutlet ducts in the housing end plate and registrable in sequence Withsaid ports in the cylinder block on rotation of the cylinder block,whereby fluid can be transferred by reciprocation of said pistons fromthe inlet duct, via said transfer ports and said ports and cylinders inthe port block, to the outlet duct, said pressure balancing meanscomprising means for distributing pressure fluid in opposed relationshipthrough an annular surface of the port plate symmetrically about saidaxis, a pressurefluid connect-ion to said means for distributingpressure fluid, restrictor means in said pressure-fluid connection forcontrolling the pressure in said means for distributing pres suredepending on the rate of escape of fluid therefrom, a radial locatingmember radially fixed relative to said axis having a circular referencesurface adjacent to and spaced from said annular surface of the portplate for controlling the rate of escape of fluid from said means fordistributing pressure fluid in dependence on the spacing of said annularsurface from the reference surface.

References Cited in the file of this patent UNITED STATES PATENTS1,867,308 Durner July 12, 1932 2,546,583 Born Mar. 27, 1951 2,571,377Olah Oct. 16, 1951 2,642,809 Born, et al June 23, 1953 2,646,754Overbeke July 28, 1953 2,733,666 Poulos Feb. 7, 1956 2,753,802 OmohundroJuly 10, 1956 2,847,938 Gondek Aug. 19, 1958 2,948,229 Brundage Aug. 9,1960 2,956,512 Brund-age Oct. 18, 1960 2,972,962 Douglas Feb. 28, 1961

1. A SWASH PLATE TYPE HYDRAULIC MACHINE COMPRISING A STATIONARY SWASHPLATE, A ROTATABLE CYLINDER BLOCK, A PLURALITY OF PISTONS RECIPROCABLEIN CYLINDERS IN SAID CYLINDER BLOCK UNDER CONTROL OF THE SWASH PLATE,PORTS IN A FLAT FACE OF THE CYLINDER BLOCK COMMUNICATING WITH THECYLINDERS THEREIN, A STATIONARY PORT BLOCK HAVING A FLAT FACE, INLET ANDOUTLET PORTS IN SAID FLAT FACE OF THE PORT BLOCK REGISTRABLE WITH THEPORTS IN THE CYLINDER BLOCK, A PORT PLATE INTERPOSED BETWEEN THE FLATFACES OF THE CYLINDER BLOCK AND THE PORT BLOCK AND HAVING APERTURESPERMITTING THE PASSAGE OF FLUID BETWEEN THE PORTS IN THE CYLINDER BLOCKAND THE PORTS IN THE PORT BLOCK, SAID PORT PLATE HAVING OPPOSITE FLATFACES OF SUNSTANTIALLY EQUAL AREA AND CLEARANCES BEING PROVIDED BETWEENSAID OPPOSITE FACES AND THE FLAT FACES OF THE CYLINDER BLOCK AND THEPORT BLOCK WITHIN THE LIMITS OF WHICH CLEARANCES THE PORT PLATE IS FREETO MOVE AXIALLY, WHEREBY PRESSURE FLUID FROM SAID PORTS MAINTAINS AFLUID FILM ON